There is a great need for tetrahydropteridine C6-stereoisomers. For example, N5-formyl-tetrahydrofolic acid (also known as leucovorin) is being used to potentiate the effects of 5-fluoro-uracil in the treatment of several forms of cancer. Another use is the regime of leucovorin "rescue" following high dose methotrexate in cancer chemotherapy or for immunosuppression. Leucovorin is also co-administered with trimetrexate for the treatment of Pneumocystis carinii pneumonia which is common in AIDS patients. Megaloblastic anemia and dihydropteridine reductase deficiency are also treated with leucovorin. Yet because of the unavailability of the natural (6S)-isomer, these therapies have been performed with (6R,S)-N5-formyl-tetrahydrofolic acid. The unnatural (6R)-isomer cannot perform the functions of the natural isomer. Furthermore, the unnatural isomer of N5-formyl-tetrahydrofolic acid is cleared much more slowly from the blood. Several hours after administration the concentration of unnatural isomer can exceed that of the natural isomer by two orders-of-magnitude. Enzymes, such as thymidylate synthase and glycinamide ribonucleotide formyl transferase, are inhibited by the unnatural isomers of their tetrahydrofolate cofactors. Further, it is well known that high concentrations of folates can cause kidney damage.
Another natural tetrahydropteridine, (6R)-tetrahydrobiopterin ((6R)-BH.sub.4) is required for metabolic control of phenylalanine levels, and for the biosynthesis of serotonin and the catecholamine neurotransmitters/hormones. In these roles it functions as the natural cofactor for the three aromatic amino acid hydroxylases. Recently (6R)-BH.sub.4 has been found to be important in the regulation of blood pressure and in the immune response as a participant in the formation of nitric oxide from arginine. The importance of the C6-chirality of BH.sub.4 for its biological activity has been well demonstrated. Cofactor replacement therapy for children having a defect in the tetrahydrobiopterin biosynthetic pathway now uses exclusively the costly natural (6R)-isomer. There are several other non-therapeutic uses for specific C6-stereoisomers of tetrahydropteridines, for example as affinity ligands for enzyme purification. Pure unnatural isomers of tetrahydropteridines are also required for experimental elucidation of their mechanism of toxicity.
Since prior to this invention there has been no method for the synthesis of important tetrahydropteridine C6-stereoisomers, other approaches have been used for their production, but with limited success. Oxidized pteridines have been reduced with chiral reagents but this gives only a low enantiomeric excess. The natural isomers of tetrahydrofolic acid and tetrahydrobiopterin have been produced by enzymatic reduction of their respective 7,8-dihydropterins with dihydrofolate reductase, but this is useful only for small quantities. The enantiomers of 6-methyl-tetrahydropterin have been resolved by fractional crystallization as the tartrate salt. Tetrahydrofolic acid (as a menthylchloroformate derivative), leucovorin, and pentacetyl-tetrahydrobiopterin, all of which are diastereomers, also have been resolved by fractional crystallization. These methods suffer considerable loss of yield either in the multiple crystallizations required to reach acceptable enantiomeric purity and/or in the subsequent chemical steps needed to liberate the desired product from a derivative. Tetrahydrobiopterin, 5,10-methylene-tetrahydrofolic acid and leucovorin have been chromatographically separated into individual C6-isomers. However, this approach is very limited with respect to scale. Considering the deficiencies of all the reported methods in filling such an important need, it is evident that there existed no obvious synthesis for 6-monosubstituted tetrahydropteridine C6-stereoisomers.